Endothelin Receptor Antagonists and Agonists

Endothelins and their receptors are increasingly important in embryological development, with various expression patterns in tissues and organs involved in numerous physiological and pathophysiological processes. Numerous Phase I-III clinical trials are exploring new applications of endothelin A and endothelin B receptor antagonists and agonists. To support organizations and studying the endothelin system, Bachem offers a selection of endothelin and related peptides.

Background

The endothelins are a family of 21-amino acid vasoconstrictor peptides comprising three isoforms, endothelin-1 (ET-1), endothelin-2 (ET-2), and endothelin-3 (ET-3). They mediate their actions via two receptors, ET-A and ET-B, found in several mammalian species.

ET-1 is the most potent mammalian vasoconstrictor identified to date with EC50 values in the subnanomolar range and is likely to have a role in controlling perfusion in every organ in the body.

Endothelial cells are the major sources of ET-1, along with epithelial cells, macrophages, fibroblasts, cardiac myocytes, and neurons. ET-1 is synthesized in a constitutive pathway for continuous release and in a second pathway involving most probably degranulation from special storage granules upon external stimulus.

ET-2 and ET-3 differ from ET-1 in two and six amino acids, respectively. The members of the endothelin family contain four cysteine residues, which form two intra-chain disulfide bridges linking cysteine residues 1 to 15 and 3 to 11.

ET-2 is expressed in intestinal epithelial cells and at lower levels in the heart. ET-3 is expressed in brain neurons, kidney, and intestinal epithelial cells. The endothelins show very close structural and functional relationship to the snake venom peptides, sarafotoxins, which comprise four isoforms, S6A, S6B, S6C and S6D (or SRTX-A, -B, -C and –D).

Sequential proteolytic processing of preproendothelins creates mature endothelins. Removal of the signal peptide and further processing results in the production of physiological inactive precursors, the ‘big endothelins’. These are further cleaved by endothelin converting enzymes (ECE) to form active endothelins. ECE are type II membrane proteins with two known members, ECE-1 and ECE-2, and various isoforms (ECE-1 A-D and ECE-2 A-C). The synthesis and secretion of ET-1 is increased by various growth factors, cytokines and vasoactive factors. Hypoxia and shear stress have also been shown to increase ET-1 release from endothelial cells. ET-1 expression is suppressed by several factors including atrial natriuretic peptide, nitric oxide, prostacyclin, and heparin. ET-1 peptide is rapidly cleared from the plasma with a half-life of several minutes.

Clinical applications of endothelin receptors

Endothelins and their receptors are increasingly important in embryological development, with various expression patterns in tissues and organs involved in numerous physiological and pathophysiological processes. These include pulmonary arterial hypertension (PAH), atherosclerosis, myocardial infarction, and vasospasms after subarachnoid hemorrhage as well as risk of hypertension and atherosclerotic vascular disease in overweight and obese people. The ET-1 system furthermore could be an important component in the pathogenesis of atrial fibrillation (AF), a disease, which basically resembles an abnormal heart rhythm.

Endothelins and their receptors are involved in the control of kidney functions such as renal blood flow, water and sodium excretion, and acid-base balance. Obviously, they also play a role in numerous cancer-relevant processes. ET-1 might influence the growth and progression of a variety of tumors such as prostatic, ovarian, colorectal and brain tumors as well as cervical carcinomas and melanomas.

Pain signaling is thought to occur by binding of ET-1 to ET-A receptors, localized on nociceptors. Therefore, ET-A antagonists could serve as pain relieving agents in many acute diseases, but also for patients suffering chronic pain. ET-1 also enhances the expression of adhesion molecules on vascular endothelial cells and stimulates the aggregation of polymorphonuclear neutrophils, contributing to inflammation and endothelial dysfunction.

Other diseases, for which the endothelin system is of high medical interest, are preeclampsia, a disease for which effective therapeutic options are still not available, Hirschsprung’s disease, a multigenic developmental disorder, and eventually sepsis.

ET-A and ET-B belong to the superfamily of the seven transmembrane G-protein coupled receptors (GPCRs) with an extracellular N-terminal and an intracellular C-terminal domain. ET-A and ET-B receptors have contrasting functions under physiological conditions. The stimulation of ET-A receptors in the smooth muscle leads to constriction, whereas activation of the ET-B receptors in the endothelial cells leads to the release of vasodilators.
Modified endothelins and peptides acting on endothelin receptors in general are indispensable tools to investigate the processes and mechanisms associated with these GPCRs.

ET-1 and ET-2 have similar binding affinities for the ET-A, but differ slightly in their affinities for ET-B. ET-3 preferentially binds to ET-B receptor and hence can be considered as moderately selective for ET-B. The affinity of ET-1 to both, ET-A and ET-B resembles an autocrine feedback mechanism, since both receptors are counter-acting. This mechanism presumably is important for the cardiovascular homeostasis. Obviously, low levels of ET-1 promote vasodilatation, whereas higher and pathophysiological concentrations increase blood pressure and total peripheral vascular resistance.

The endothelin A and endothelin B receptors mediate the actions of endothelin and both receptor subtypes are targets for developing therapies for human diseases. Three endothelin-receptor antagonists, bosentan, ambrisentan and macitentan, are already approved for the treatment of pulmonary arterial hypertension (PAH). In the future, endothelin receptor antagonists and agonists might find a place in the treatment of other diseases such as diabetic nephropathy, cerebral vasospasm, cancer and diabetic ketoacidosis.

Endothelin receptor antagonists in clinical development

There are many endothelin receptor antagonists in various phases of clinical development as shown below:

Endothelin Receptor Antagonists and Agonists in Phase I-III Clinical Development (data from Medtrack)

Phase III Candidates

Xinlay (atrasentan hydrochloride), a pyrrolidine derivative, is an endothelin A receptor antagonist that AbbVie is developing for the treatment of diabetic nephropathy. Abbott Laboratories was developing Xinlay for the treatment of prostate cancer but the U.S. Food and Drug Administration rejected the company’s New Drug Application for the prostate cancer indication back in 2005. Development continued for the diabetic neuropathy indication and in 2013, AbbVie initiated a Phase III clinical study to assess the effects of atrasentan on renal outcomes in patients with type 2 diabetic nephropathy while they continue to be treated with the current standard of care.

Actelion Pharmaceuticals is developing Pivlaz (clazosentan) for the reversal of vasopasm associated with aneurysmal subarachnoid hemorrhage. In 2017, Acetlion completed a Phase II study to assess the efficacy and safety of clazosentan in reversing cerebral vasospasm in patients with aneurysmal subarachnoid hemorrhage treated by surgical clipping or endovascular coiling.

Retrophin is developing RE021 (sparsentan), a dual-acting endothelin and angiotensin receptor antagonist. Sparsentan is being developed for the treatment of diabetic nephropathy, a rare kidney disorder known as focal segmental glomerulosclerosis (FSGS), immunoglobulin A nephropathy and hypertension. Sparsentan has orphan drug designation from the U.S. Food and Drug Administration and European Commission for FSGS. The product is in a Phase II/III clinical trial for treatment of FSGS.

Bachem endothelin products

Clinical trials are exploring new applications of endothelin A and endothelin B receptor antagonists and agonists. To support organizations and studying the endothelin system, Bachem offers a selection of endothelin and related peptides.

The Bachem online shop sells a full range of variously derived ET-1, ET-2 and ET-3 strains along with endothelin antagonists that include:

• BQ-123 (H-1252)
• BQ-610 (H-4914)
• BQ-788 (H-24920
• BQ-788 ammonium salt (H-8242)
• BQ-788 sodium salt (H-8152)
• Cyclo(-D-Glu-Ala-D-allo-Ile-Leu-D-Trp) BE-18257B (H-8405)
• Cyclo(-Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp)-Trp-Phe-Phe-Asn-Tyr-Tyr-Trp-OH RES-701-1
• (H-2508)
• Cyclo(-Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp)-Trp-Val-Tyr-Phe-Ala-His-Leu-Asp-Ile-Ile-Trp-OH (H-4074)
• Cyclo(-D-Ser-Pro-D-Val-Leu-D-Trp) JKC-302 (H-3064)
• Endothelin-1 (11-21) trifluoroacetate salt IRL-1038 (H-1658)
• Acetyl-(D-Trp¹⁶)-Endothelin-1 (16-21) & Ac-[DTrp16] Endothelin-1 (16-21), human (H-8850)
• JKC-301 (H-3008)

Resources

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